Among oxidation dyes commonly used in oxidation hair dyes, those having two or more amino groups per molecule (for example, toluene-2,5-diamine, p-phenylenediamine, m-phenylenediamine) are disadvantageous in that they are liable to be oxidized by oxygen in the air. As a result, these dyes suffer from a decrease in purity or assume a black or tar-like appearance upon storage, which makes them unsuitable for use.
In order to improve the stability of these oxidation dyes, it has been attempted to use them in the form of salts, such as sulfates or hydrochlorides. Since oxidation dyes are generally used under neutral or alkaline conditions, however, it is necessary to neutralize such a dye in the form of a salt by adding a basic material to such a dye composition. This neutralization step results in the formation of a large amount of salt which lowers the solubility of other components (for example, dyes, oils) or lowers the emulsification stability of the composition. Thus, the formulation of such a dye composition is greatly restricted.
Accordingly, it has been required to use such a dye not in the form of a salt but in the non-salted form or salt free form to improve the stability thereof.
On the other hand, it is very convenient to prepare a concentrate of such a dye which can be easily formulated into a dye composition at use. However, the art has not believed it to be practically possible to prepare such a concentrate since said dye is highly unstable, as earlier described.
Under these circumstances, we conducted extensive studies in order to solve the above-mentioned problems. As a result, we found that a dye concentrate can be obtained by blending such a dye in the non-salt form or salt free form with a reducing agent, thus reaching the present invention.